The present invention relates to the field of Hall effect thrusters.
The invention relates more particularly to a Hall effect thruster having an annular channel, an anode, an injection circuit, a magnetic circuit, and a cathode. The annular channel is defined by an outer wall and an inner wall that are coaxial around a central axis, and it presents a downstream end that is open and an upstream end that is closed. The anode is situated at the upstream end of the annular channel. The injection circuit is suitable for injecting a propulsion gas, e.g. xenon, into the annular channel. The magnetic circuit is suitable for generating a magnetic field at the downstream end of the annular channel. The cathode is situated at the outside of the downstream end of the annular channel.
In the present context, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the propulsion gas in the direction defined by the central axis of the annular channel.
Typically, in the operation of such a Hall effect thruster, electrons are emitted by the cathode and attracted towards the anode at the end of the annular channel, which electrons are trapped by the magnetic field in spiral trajectories between the two walls, thereby forming a virtual cathode grid. Electrons escaping towards the anode from this magnetic enclosure enter into collision with atoms of the propulsion gas that is injected into the annular channel by the injection circuit, thereby creating an ionized plasma.
The positive ions of the plasma are accelerated by the electric field that exists between the anode and the virtual cathode grid formed by the cloud of electrons trapped by the magnetic field at the open end of the annular channel. Since the mass of each positive ion is much greater than the mass of an electron, the trajectory of the positive ions is hardly affected by the magnetic field. The ions of this plasma jet are finally neutralized downstream from the magnetic field by electrons emitted by the cathode or that have been produced by ionizing the plasma.
Hall effect thrusters were initially used in attitude and orbit control systems (AOCSs) for space vehicles, and in particular for the AOCSs of geostationary satellites. Hall effect thrusters enable a specific impulse (Isp) to be obtained that is very high, being of the order of 1500 seconds (s), thus enabling the attitude and/or the position of the vehicle to be controlled very accurately with mass and complexity that are significantly smaller than when using conventional systems relying on inertial devices such as reaction wheels, for example, in combination with chemical thrusters for desaturating them.
Nevertheless, a Hall effect thruster with high specific impulse can normally only achieve thrust that is very low. Consequently, AOCSs incorporating Hall effect thrusters are conventionally associated with chemical thrusters for performing certain fast maneuvers, such as positioning or orbit transfer. That nevertheless presents the drawback of increasing the overall cost and complexity of the space vehicle, to the detriment of its reliability.
When performing characterization tests, and also in the SMART-1 experiment, it has been established that Hall effect thrusters can operate not only in a high specific impulse mode using the propulsion gas at a low flow rate and using a high electric voltage between the anode and the cathode, but that they can also be used, alternatively, in a high thrust mode, with a flow rate that is large and an electric voltage that is moderate. Nevertheless, the stability of the plasma jet and the efficiency of the thruster depend, amongst other things, on the density of the plasma in the annular channel. Consequently, present thrusters are optimized for a single mode of operation. Thus, replacing chemical thrusters for positioning and for orbit transfer would normally require Hall effect thrusters that are designed to operate in a high thrust mode, in addition to Hall effect thrusters of high specific impulse in the AOCS. The complexity of the space vehicle would thus not be reduced significantly.
In U.S. Pat. No. 7,500,350 B1, a Hall effect thruster is disclosed that has an annular channel presenting a downstream end that is open and an upstream end that is closed, an electric circuit, an injection circuit for injecting a flow of propulsion gas into the annular channel, and a magnetic circuit for generating a magnetic field at the downstream end of the annular channel. The electric circuit comprises an anode situated at the upstream end of the annular channel, a cathode downstream from the downstream end of the annular channel, and an electric voltage source between said anode and said cathode. The annular channel is defined by an inner wall and an outer wall that are coaxial around a central axis. In order to maintain a gap that is approximately constant between the inner and outer walls in spite of them being gradually eroded away during the operation of the thruster, the inner wall and/or the outer wall are movable in the axial direction, and the thruster also has an actuator for moving said inner wall and/or said outer wall. Nevertheless, that document does not mention how to operate the Hall effect thruster both in high thrust mode and in high specific impulse mode.